[9.x] Allow for accessing cookies via TestResponse

[inxilpro]

TestResponse::getCookie() is currently protected, and is only used internally for cookie assertions. This PR exposes this method and handles the decryption of cookies.

This is particularly useful if a cookie has an unpredictable value which is needed later in your test.

Before:

$cookie = collect($response->headers->getCookies());
  ->first(function(Cookie $cookie) {
    return 'my_cookie' === $cookie->getName();
  });

$value  = CookieValuePrefix::remove(Crypt::decrypt($cookie->getValue(), false));

After:

$value = $response->getCookie('my_cookie')->getValue();

It's worth noting that TestResponse::getCookie() already exists and is used almost exactly like the "Before" use-case. This simply makes that functionality available to application tests, and adds framework tests for it.

This also adds a bunch of test cases for cookie assertions, which were missing before.

In one test I discovered a bug which I can also submit as a separate PR to 8.x. Right now assertCookieNotExpired() doesn't properly handle "session" cookies (i.e. cookies where the expiration is 0). This PR fixes that bug and adds a test for it.

[inxilpro]

StyleCI is giving me a 500 error when I try to load the check. I'll check back in a bit and apply any necessary patches.

[inxilpro]

The two bugs that came up (that I'll submit separately to 8.x) are:

• assertCookieNotExpired incorrectly triggers when expiration is set to 0
• assertCookieExpired incorrectly passes when expiration is set to 0